I haven't seen a source for new parts. Those things are gold.
Ebay or used arcade websites are your best bet.
Andy Geezer usually has that stuff, but he's in the UK.
http://triplemoonstar.brinkster.net/theshed/default.asp?maincat=-99Video lottery terminal (VLT) buttons are about the same size. They aren't a drop in replacement. You'd have to ditch the circuit board behind the current buttons and replace them all. There might be a way to graft the front of one onto the current button.
Happ sells them for big $$$.
There was a guy selling the rectangular ones as "Daytona replacements" on ebay fairly cheap, but he's down to only 3 colors now.
(I bought three sets
)